1. Field of Invention
This invention discloses an adsorbent material fabricated into a self-supported coherent sheet and configured for use as a parallel passage contactor.
2. Prior Art
Traditional mass transfer devices for adsorption process include monoliths (e.g. honey combs), cloth (e.g. activated carbon fiber cloth) and packed bed of adsorbent particles. The packed bed is cheap and versatile, but quite inefficient in operation at high flow rate regimes due to the high costs associated to the large internal pressure drop. Calculations by Ruthven and Thaeron (in Gas. Sep. Purif. vol. 10, (1996) p. 63) have shown that a significant improvement in the mass transfer/pressure drop characteristics over the packed bed configuration could be achieved with parallel passage contactors. These are mass transfer devices in which the gas passes in laminar flow through straight channels between equally spaced parallel sheets of adsorbent. Rapid mass transfer enables rapid cycling and smaller devices. One application of parallel passage contactors is Pressure Swing Adsorption (PSA).
PSA has become of interest for small-scale gas separation applications because of its potential for high separation performance (product purity and recovery) compared with other gas separation technologies. Depending on the actual mechanism, PSA separations could be categorized as equilibrium or kinetically (diffusion) controlled. An example for the first category is separation of air on zeolite 5 Å into almost pure nitrogen and oxygen streams based on differences in equilibrium adsorption isotherms between nitrogen and oxygen. An example for the second group is the same separation carried out on zeolite 4 Å, where the mechanism is based on the differences in diffusion rates between nitrogen and oxygen, which have different effective kinetic diameters (3.46 Å and 3.64 Å, respectively). While the equilibrium based PSA separation has been relatively well established theoretically and already commercialized for some applications, the diffusion-induced PSA still needs theoretical development and is not fully commercialized. The reader is referred to several recent publications such as by Shin and Knaebel, in AlChE Journal, vol. 33, p. 654 (1987), and vol. 34, p. 1409 (1988); by Chung and others, in Computers Chem. Engn. Vol. 22, Suppl., p. S637 (1998); and to the “Pressure Swing Adsorption” monograph book by Ruthven, Farooq and Knaebel, VCH Publisher, 1994.
Another way for optimization of PSA in terms of enhancing the adsorbent productivity at equal recovery and product purity parameters is through shortening the cycle times. Enhanced adsorbent productivity results in reducing the cost and foot print size of PSA beds. When the intra particle diffusion limits the rate of mass transfer in PSA, one way to shorten the cycle times is by using adsorbent with very small particle sizes. This was demonstrated first in U.S. Pat. No. 4,194,892, where relatively small particles of adsorbent were used in a packed bed configuration at cycle times of less than 30 seconds, with substantially higher product recovery than in previous art. U.S. Pat. No. 4,354,859 demonstrated a further increase in productivity by executing rapid cycle PSA with two pistons operating out-of-phase at the ends of the adsorption column.
However, the gas separation efficiency in rapid cycle PSA, as described by U.S. Pat. Nos. 4,194,892 and 4,354,859, is limited by the pressure drop in the randomly packed bed column. To circumvent this, Farooq, Thaeron and Ruthven (Sep. Pur. Tech., vol. 13 (1998) p. 181–193) suggested combining piston-driven rapid cycle PSA with parallel passage contactors, thus providing an economical solution to older separation technologies such as air drying, air separation, and VOC removal. Based on numerical simulation models developed by Ruthven and Thaeron (Gas Sep. Purif. vol. 10 (1996) 63–73), for example, a parallel passage contactor with sheet thickness of about 500 to 800 μm and sheet spacing of about 100 μm should be well suited to rapid cycle adsorption processes for CO2/N2 separation. The adsorbent described by Ruthven and Thaeron was activated carbon fiber (ACF) sheet with fiber diameters of 10–15 μm. For this adsorbent characteristic length, the optimal cycle frequency was 10–20 rpm, the rate being limited by the inter particle, macro pore diffusion. However, it became evident for those who tried to use carbon fiber adsorbent in woven or non-woven form in rapid cycle PSA systems that a great disadvantage of these materials is that they are not dimensionally stable.
Further increase in cycle frequency and thus more performance improvement of the separation process is possible in principle by using even smaller adsorbent particles (about 10 μm in size). Problems with small particles in packed beds subjected to high flow velocities include particle break up, particle attrition from the bed, and particle fluidization. U.S. Pat. No. 6,176,897 teaches a high frequency pressure swing adsorption system in which granular adsorbent beds are replaced by a high surface area adsorbent monolith or layered support, with adsorbent elements formed of layered or laminated sheet materials using fibrous reinforcements (such as glass, carbon or kevlar fibers) which support zeolite loaded composites in adsorbent sheets. However, the availability of materials that could be successfully used for fabrication of such adsorbent structures is limited. Also, the use of reinforcement materials limits the adsorption capacity per volume of adsorption bed, because a relatively large fraction of the adsorbent bed volume, associated with the reinforcement structures, is not effectively used for adsorption.
It was demonstrated that inorganic adsorbent particles with sizes in the range of tens to hundreds of nanometers (also called adsorbent nanoparticles) have enhanced adsorption and chemical surface reactivity due to the very high ratio of surface atoms to bulk atoms. For more information, the reader is directed to the recently published book entitled “Nanoscale Materials in Chemistry” (Wiley, 2001) by Klabunde. However, integrating adsorbent nanoparticles into usable sorptive materials has been a challenge so far. Efficient means for binding, stabilizing or incorporating adsorbent particles with sizes in the nanometer range in structures that can be used for adsorption and separation applications are deemed necessary.
Recently, sorptive materials based on polytetrafluoroethylene (PTFE) matrix have been described in the patent literature. U.S. Pat. Nos. 4,810,381 and 4,906,378 describe a chromatographic sorptive material composed of PTFE fibril matrix and non-swellable adsorbent particles enmeshed in the matrix U.S. Pat. Nos. 4,153,661 and 5,071,610 disclose manufacturing methods and uses of composite sheet materials comprised of fine, non-swellable adsorbent particles held by a fibrillated polymer matrix, and methods for the control of internal porosity. The resulting sheet is extremely pliable and it is said to be useful as an electronic insulator or semi permeable membrane. U.S. Pat. No. 4,373,519 discloses a composite wound dressing comprising hydrophilic absorptive particles enmeshed in a PTFE matrix. U.S. Pat. Nos. 4,565,663 and 4,460,642 disclose water swellable composite sheets having a PTFE matrix in which are enmeshed swellable hydrophilic sorptive particles. However, sorptive materials obtained by enmeshing particulate sorbents, in a fibrillated PTFE matrix with specification for use as a parallel passage contactor have not been disclosed.
Self-supported porous membranes obtained by compacting micron-size carbon particles and fibrillated PTFE could also be used as porous electrodes in electrochemical applications. The U.S. Pat. No. 4,585,711 teaches a hydrogen electrode for a fuel cell obtained by roll compaction of granular PTFE and platinum-covered carbon black particles. The U.S. Pat. No. 4,379,772 disclosed a method for forming an active layer electrode for fuel cells in which granules of active carbon are mixed with fibrillated PTFE and rolled into a self-supported, coherent sheet form. U.S. Pat. No. 4,468,362 discloses a method for preparing a self-sustained electrode-backing layer with excellent electrical conductivity through dispersing PTFE particles and finely divided carbon black particles (50 to 3000 Å). U.S. Pat. No. 4,500,647 teaches the preparation of three-layer matrix electrodes for fuel cell or other electrochemical applications in which active carbon particles are present within an unsintered carbon black-fibrillated PTFE material. U.S. Pat. No. 5,636,437 discloses a fabrication method of solid carbon porous electrodes from various carbon powders and thermoset resin binders. These un-reinforced, self-supporting sheets have not been specified for use as a parallel passage contactor. The prior art is limited to adsorbent cloths or reinforced sheets for parallel passage applications.